1. Field of the Invention
This invention relates to electronic circuits for automatically connecting and disconnecting a battery backup supply.
2. Description of the Prior Art
In the course of acquiring, processing and displaying information a variety of solid state memory devices may be utilized. Some types of these solid state memory devices, such as a random access memory (RAM) require a continuous supply of power thereto in order to prevent a loss of stored information. Consequently, an interruption in power to a RAM will destroy the stored information.
Various methods have been used to solve this problem. One approach has been to power the memory device directly from a storage cell such as a lead-acid battery or a nickel-cadmium battery. The main disadvantage of this approach is the constant requirement for recharging and/or replacing the battery.
A method which avoids the above problem utilizes a sensing circuit which connects the battery to the memory device only upon the loss of power from the regulated power supply. This method, however, has the disadvantage permitting the battery to completely discharge if the regulated power supply does not become operative within a certain period of time. This certain period of time is dependent upon the capacity of the battery (commonly expressed in units of ampere-hours) and the current drawn by the circuit connected to the battery. The disadvantage of this method is that even though the memory device will lose its stored information when the voltage of the battery drops below a certain level, the battery continues to discharge. With many types of batteries, such as lead-acid types and nickel-cadmium types, the cell cannot be recharged if it has been completely discharged repeatedly.
Accordingly, it is desireable to provide a method and apparatus for automatically connecting a battery backup supply upon the failure of a main d.c. power supply, in addition to preventing a complete discharge of the battery backup supply.